The present disclosure relates generally to a method for plaque characterization and in particular, to a method for using multiple x-ray energy level absortiometry techniques for plaque characterization in cardiac applications.
In at least one known computed tomography (CT) imaging system configuration, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system, wherein the X-Y plane is generally referred to as an “imaging plane”. An array of radiation detectors, wherein each radiation detector includes a detector element, is within the CT system so as to receive this fan-shaped beam. An object, such as a patient, is disposed within the imaging plane so as to be subjected to the x-ray beam wherein the x-ray beam passes through the object. As the x-ray beam passes through the object being imaged, the x-ray beam becomes attenuated before impinging upon the array of radiation detectors. The intensity of the attenuated beam of radiation received at the detector array is responsive to the attenuation of the x-ray beam by the object, wherein each detector element produces a separate electrical signal responsive to the beam attenuation at the detector element location. These electrical signals are referred to as x-ray attenuation measurements.
In addition, the x-ray source and the detector array may be rotated, with a gantry within the imaging plane, around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. A group of x-ray attenuation measurements, i.e., projection data, from the detector array at one gantry angle is referred to as a “view”. A “scan” of the object comprises a set of views made at different gantry angles during one revolution of the x-ray source and the detector array. In an axial scan, the projection data is processed so as to construct an image that corresponds to a two-dimensional slice taken through the object.
One method for reconstructing an image from a set of projection data is referred to as the “filtered back-projection technique”. This process converts the attenuation measurements from a scan into discrete integers, ranging from −1024 to +3071, called “Hounsfield Units” (HU) or CT HUs. These CT HU's are used to control the brightness of a corresponding pixel on a cathode ray tube or a computer screen display in a manner responsive to the attenuation measurements. For example, an attenuation measurement for air may convert into an integer value of −1000 HU's (corresponding to a dark pixel) and an attenuation measurement for very dense bone matter may convert into an integer value of +1000 HUs (corresponding to a bright pixel), whereas an attenuation measurement for water may convert into an integer value of 0 HU's (corresponding to a gray pixel). This integer conversion of attenuation value to various shades of gray allows a physician or a technician to determine the density of matter based on the intensity of the computer display.
A strong and graded association exists between coronary calcification and cardiovascular disease. Coronary calcification scores have been used to measure atherosclerotic plaque burden and the extent of angiographically detected coronary artery disease and as a screening tool to predict the extent of cardiovascular disease and to develop risk factor for future cardiac event such as heart attack. Larger, high-density calcified plaques are associated with stable coronary events. Certain plaques are more vulnerable to detaching off the walls of the artery and prompting a blood clot, which travels to the heart and can cause a sudden unanticipated fatal heart attack. This so-called vulnerable plaque is soft because of its high lipid, or fat, content. Soft plaques, also referred to as non-calcified plaques, are not highly calcified and hence calcium scores in such subjects may be minimal indicating that the subject is normal. The effectiveness of a screening tool lies in the early detection of disease and current calcium screening tests may not be effective because they do not account for the measurement of soft plaques.